In certain electrostatographic imaging and recording processes, for instance, in electrophotographic copying processes, an electrostatic latent image formed on a photoconductive surface is developed with a thermoplastic toner powder which is thereafter fused to a substrate. The fusing member can be a roll, belt or any surface having a suitable shape for fixing thermoplastic toner powder images to a substrate. The fusing step commonly consists of passing the substrate, such as a sheet of paper on which toner powder is distributed in an imagewise pattern, through the nip of a pair of rolls. At least one of the rolls is heated and in the case where the fusing member is a heated roll, a smooth resilient surface is bonded either directly or indirectly to the core of the roll. Where the fusing member is in the form of a belt it is preferably a flexible endless belt having a smooth, hardened outer surface which passes around the heated roller. A persistent problem in this operation is that when the toner is heated during passage through the rolls it may tend to adhere not only to the paper but also to the fusing member which contacts it. Any toner remaining adhered to the member can cause a false offset image to appear on the next sheet that passes through the rolls and can also degrade the fusing performance of the member. Another problem may occur as a result of continued heating causing degradation of the member surface which results in an uneven surface and defective patterns in thermally fixed images.
Toner fusing rolls are composed of a cylindrical core which may contain a heat source in its interior, and a resilient covering layer formed directly or indirectly on the surface of the core. A thin layer of a suitable primer is advantageously coated on the surface of the core in order to improve bonding of the layer. Roll coverings are commonly made of fluorocarbon polymers or silicone polymers, such as poly(dimethylsiloxane) polymers, of low surface energy which minimizes adherence of toner to the roll. Frequently release oils composed of, for example, poly(dimethylsiloxanes) are also applied to the roll surface to prevent adherence of toner to the roll. Such release oils may interact with the roll surface upon repeated use and in time cause swelling, softening and degradation of the roll. Silicone rubber covering layers which are insufficiently resistant to release oils and cleaning solvents are also susceptible to delamination of the roll cover after repeated heating and cooling cycles.
Toner fusing belts are composed of a continuous flexible material having superior resistance to heat and a smooth surface. The belt substrate can be metallic or polymeric. The surface of the belt is composed of a thinly coated, low surface energy polymer such as a fluorocarbon or a silicone resin. There is a need for coating compositions which adhere strongly to the belt and form a hard, tough surface which is resistant to wear and cracking. The surface should also be resistant to cleaning solvents and fluids.
In electrostatographic imaging processes dry developers can be used to form an image on a receiving surface such as a sheet of paper. Dry developers usually comprise a toner powder and carrier particles. Carrier particles and toner particles have different triboelectric values. As the developer mixture is agitated the particles rub together and the toner and carrier particles acquire opposite electric charges and cling together. In the subsequent development step the somewhat higher opposite charge of the electrostatic latent image draws the colored toner from the carrier and develops the image. Frequently, various addenda are used to improve the properties of the toner and carrier particles.
Toners comprise, as a major component, the binder, and as minor components a colorant and a charge control agent. The binder can be any resin which has properties suitable for dry toners. Many such resins are known but thermoplastic resins which are fixable by fusing are especially useful. When a dry toner powder image is transferred from one surface to another defects in the image can occur. It is known from U.S. Pat. No. 4,758,491 that by the addition of low surface energy liquid or solid addenda, especially polymers containing organopolysiloxane segments, many of these defects can be alleviated.
Carrier particles comprise magnetizable irregular particles which are usually coated with a film of a polymeric material which helps develop the triboelectric charge and aids the transfer of the toner. The coating material must adhere well to the carrier particle because the toner charge declines as the polymer wears off. Polymers with low surface energy properties are especially useful for coating carrier particles.